A silver halide emulsion, particularly for feeding an aqueous silver nitrate solution and an aqueous halide solution with high accuracy into an aqueous protective colloidal solution in a precipitation vessel for preparing silver halide crystals.
The most important process in manufacturing a photographic emulsion is the one to form silver halide crystals. Precisely controlled size and amount (its distribution) of the silver halide crystals are main factors to give a good quality to the photographic emulsion, which leads to an improvement of photographic materials.
One of conventional way to form silver halide crystals was that an aqueous silver nitrate solution is added into a precipitation vessel containing the mixed solution of the aqueous halide salt solution and an aqueous protective colloidal solution. The ways of adding the aqueous silver nitrate solution are, for examples, to use a pressure head formed between a level of the aqueous silver nitrate solution (higher position) and that of the mixed solution in the precipitation vessel (lower position), and to use pump. To control a flow rate for addition, an orifice is put in its feed line or pump is just controlled to control its discharging amount. However this way not only does not give accurate control of flow rate but is not enough to control a structure of crystal of silver halide.
U.S. Pat. No. 3,782,954, Japanese Patent Publication 41114/78 (tokko-sho 53-41114) and Japanese Patent Publication 58288/83(tokko-sho 58-58288) show the method that an aqueous silver nitrate solution and an aqueous halide salt solution are fed into a mixer installed in a precipitation vessel to obtain an improved uniformity in size of silver halide crystal grains In Japanese Laid-open Patent 138282/76 (tokkai-sho 51-138282), U.S. Pat. No. 4,026,668, Japanese Patent Publication 31454/86 (tokko-sho 61-31454), Japanese Laid-open Patent 67952/90 (tokkai-hei2-67952), U.S. Pat. No. 5,248,577 and Japanese Laid-open Patent 232611/93 (tokkai-hei 5-232611), another method is shown in which flow rates of an aqueous silver nitrate solution and an aqueous halide salt solution are controlled to keep an electric potential of a mixed solution reacting in a precipitation vessel being a predetermined value. This method could control a size of each silver halide crystal grain.
In most of the above described methods, pumps are used for feeding and adding an aqueous silver nitrate solution and an aqueous halide salt solution from their storage vessels to a precipitation vessel because those pumps have a good controllability and a good responsibility, and a lot of selections to meet user""s purpose. The pumps, however, generally have a narrow range of flow rate with high accuracy, which requires to use a plurality of pumps each of which have a different range of flow rate to cover a desired whole range of flow rate.
As mentioned above, the most important process in manufacturing a photographic emulsion is the one to form silver halide crystals.
Precisely controlled size and amount (its distribution) of the silver halide crystals are main factors to give a good quality to the photographic emulsion, which leads to an improvement of photographic materials. To obtain such silver halide emulsion, it is necessary to control the flow rate of those solutions added into the precipitation vessel with high accuracy to meet the rate required at the initial stage and at the terminal stage of adding the solutions according to a predetermined program and at the steady state according to a required flow rate at every moment, and to control precisely the total amount of the solution added
To avoid a waste of the solution which are expensive, it is desirable to consume whole the solution once prepared without leaving any of the solution or to make the feed line available in common to forming any types of a great number of emulsions. For example, at least basic nine types of emulsion are required to make color negatives, such as emulsion for blue-sensitive layer, green sensitive layer and red sensitive layer, each of which are divided to three emulsion layers, high sensitive layer, medium sensitive layer and low sensitive layer. Furthermore taking a variety of photographic speed of emulsion into consideration, the number of the types of emulsion increases a lot. To cope with manufacturing that great number of emulsion types, the aqueous silver nitrate solution and the aqueous halide salt solution have to be fed according to a variety of flow rates each prescription of emulsions required respectively. To cope with the situation by only one facility to feed the solutions, it has to have a wide range controllability in flow rate within which high accurate control is kept, for example, the ratio between the minimum flow rate and the maximum flow rate should be 1 to 10, preferably 1 to 20 or more. Laid-open Patent 146543/96 (tokkai-hei 8-146543) shows the method that can avoid not only above mentioned shortcomings of the prior arts but also another faults that the feed line using pump system always leaves some amount of the solution inside the feed line as a waste. However the method using pump cannot completely avoid a leakage from its sealing mechanism, such as mechanical seal, grand seal and lip seal, therefore it needs frequent maintenance specially with respect to its sealing, which would be a troublesome operation because an aqueous silver nitrate solution does harm to a skin of human. Laid-open Patent 182623/87 (tokkai-sho 62-182623) shows the method using a principle of injection syringe in which a flow rate range is easily controlled by selecting a cylinder with a different bore size. In this method accuracy of flow rate fed from the cylinder depends on machining the cylinder bore to extremely close tolerances. If the volume of the cylinders are the same, the smaller bore cylinder has generally a better accuracy but it leads to a longer cylinder which is practically undesirable.
An object of the present invention is to provide a method of preparing silver halide emulsion and an apparatus there for which are capable of controlling of flow rate of solutions added in to a precipitation vessel with high accuracy over a wide range of the flow rate without using pump or cylinder systems.
According to present invention, a method for preparing silver halide emulsion by reacting an aqueous silver nitrate solution with an aqueous halide salt solution in a colloidal solution contained in a precipitation vessel comprising the steps of applying a pressure to each source of said two aqueous solutions; controlling said applied pressure and flow-resistance of each feed lines of said two aqueous solutions so that each flow rate of the two aqueous solutions to be added into the colloidal solution can be controlled.
In this method, it is preferable to control the flow-resistance in feed lines by controlling flow-path opening in the feed lines. The flow path opening is preferably controlled by a control valve varying a cross-sectional area of flow path and an actuator to actuate said control valve which are installed in said feed line. The applied pressure and the flow-resistance are preferably controlled according to a value of electric potential measured with respect to a mixed solution reacting in the precipitation vessel.
According to another aspect of the present invention, an apparatus for preparing silver halide emulsion by reacting an aqueous silver nitrate solution with an aqueous halide salt solution in a colloidal solution contained in a precipitation vessel, comprising:
a first vessel for containing an aqueous silver nitrate solution;
a second vessel for containing an aqueous halide salt solution;
a pressure controller for controlling a pressure inside the first vessel and the second vessel;
a pressure sensor installed in the vessels respectively for measuring each pressure inside the vessels to generate a signal indicating said each pressure;
a first feed line for feeding the aqueous silver nitrate solution contained in the first vessel into the precipitation vessel;
a second feed line for feeding the aqueous halide salt solution contained in the second vessel into the precipitation vessel;
a first flow meter installed in the vessels for measuring a flow rate of the aqueous silver nitrate solution;
to generate a signal indicating said flow rate;
a second flow meter installed in the vessels for measuring a flow rate of the aqueous halide salt solution to generate a signal indicating said flow rate;
a first flow path opening controller disposed in the first feed line;
a second flow path opening controller disposed in the first feed line; and
a flow rate controller for controlling said pressure controller to adjust said pressure and for controlling said flow path opening controller to adjust said flow path opening based on a transmitted pressure value signal from said pressure sensor and a transmitted flow rate value signal from said flow meter so that a predetermined flow rate can be obtained.
In this apparatus, it is preferable to use a control valve varying a cross-sectional area of flow path and an actuator to actuate said control valve as the flow-path opening controller installed in said feed lines. Preferable control valve is, for example, one having a conical or spindle-shaped valve head with a flange to sit on a valve seat, the valve head is controllably moved by an actuator mounted on the upper portion of the valve. The predetermined flow rate is preferably determined according to a value of electric potential of the mixed solution reacting each other in the precipitation vessel measured by an electric potential detector put in the mixed solution.